The subject invention relates to a magnetorheological (MR) composition for use in a magnetorheological (MR) fluid and a method of preparing the same.
The related art provides a magnetorheological (MR) composition including a plurality of metal particles for use in a magnetorheological (MR) fluid. These MR fluids are formulated by dispersing the metal particles in a hydrocarbon carrier fluid. Anti-settling, anti-friction and anti-wear agents may be added to the carrier fluid. A surfactant may also be added to the carrier fluid following dispersion of the metal particles for xe2x80x9cin situxe2x80x9d surface treatment of the metal particles. In order to assure a desirable amount of the surfactant has reacted with the metal particles in the mixture, a large excess of the surfactant must be added. Since it is usually impractical, if not impossible, to remove, the excess, non-adsorbed surfactant remains in the mixture.
One such MR fluid, shown in U.S. Pat. No. 6,149,832 to Foister, discloses the surfactant bonded to the metal particles. However, the ""832 patent does not disclose optimizing the amount of surfactant adsorbed to the metal particles for optimal MR fluid performance. Also, the ""832 patent does not disclose the MR fluid having the non-adsorbed surfactant removed from the MR fluid. Another MR fluid, shown in U.S. Pat. No. 5,143,637 to Yokouchi et al., discloses the surfactant having a lipophilic group bonded to the metal particles. However, the ""637 patent does not optimize the MR composition for optimal MR fluid performance nor remove the non-adsorbed surfactant prior to formulating the MR fluid.
Excess surfactant, however, is known to counteract the effect of thickeners, such as fumed silica, used to stabilize MR fluids against particle settling. While it is recognized that a certain amount of surfactant is beneficial in promoting particle dispersion, excess surfactant de-stabilizes the mixture, thereby counteracting the action of the thickener.
The metal particles suitable for use as the solid phase of the MR fluids are selected for their ability to perform as xe2x80x9csoftxe2x80x9d magnetizable materials. In the context of MR fluids, this means that the solid metal particle can be magnetized to exhibit a high-induced magnetic moment under a given magnetic field, but that the magnetic moment will relax, with little or no hysteresis, when the field is removed. This magnetic property is primarily dependent upon the metal particles selected for use and their chemistry and microstructure immediately prior to formulation into the MR fluids. Proper selection of suitable metallic particles and careful handling of those particles prior to formulation are therefore important in limiting the impact of processing and environmental conditions on the particles.
Metal particles typically used in the MR fluids are available in powder form. The powder is stored under atmospheric conditions until used. For safety and economic reasons, it is not practical to maintain the powders under inert atmospheres. Most powders used in MR fluids are oxidized to some degree prior to formulation. Furthermore, due to the finely divided character of the powder, oxidation tends to affect a large surface area. The practical effect of this oxidation is to change the surface chemistry of the metal particles and degrade their saturation magnetization. This, in turn, impacts upon the strength and quality of the magnetic moment that can be created in a given MR fluid under a magnetic field.
Accordingly, it would be advantageous to provide a MR composition for use in a MR fluid having improved resistance to oxidation and increased dispersability to improve the performance of the MR fluid.
The subject invention provides a magnetorheological (MR) composition having improved resistance to oxidation and increased dispersability in a carrier fluid. The composition includes a plurality of metal particles and a surfactant including a hydrocarbon with at least one of a hydroxyl, a carbonyl, and an amine group bonded to the hydrocarbon for reacting with the metal particles. The surfactant is present in an amount such that from about 1% to about 20% of the amount of the surfactant is adsorbed to the metal particles for improving the resistance of the metal particles to oxidation and for increasing the dispersability of the magnetorheological composition in the carrier fluid.
The subject invention further provides a method of preparing the MR composition. The method includes the step of reacting the metal particles with the surfactant until about 75% to about 100% of the maximum amount of the surfactant adsorbs to the metal particles. The MR composition, according to the subject invention, is utilized in a magnetorheological (MR) fluid. The MR fluid has improved stability due to the MR composition having about 75% to about 100% of the maximum amount of the surfactant adsorbed to the metal particles.
Accordingly, it has been determined that treating the surface of the metal particles prior to formulation into the MR fluid substantially improves the performance of the MR fluid and limits oxidation of the metal particles. Further, the subject invention has the added advantage of reducing the necessity for special handling of the metal particles that pose an explosive risk. The metal particles exhibit improved dispersion properties such that no addition of surfactant to the MR fluid mixture is required while exhibiting superior stability. MR fluids prepared with the MR composition are stabilized against particle oxidation and magnetic degradation typical of currently available MR compositions.